Journal of Applied Physiology, Vol 40, Issue 5 741-744, Copyright © 1976 by American Physiological Society
Maximum exercise heart rate reduction with maturation in the rat
K. A. Corre, H. Cho and R. J. Barnard
Maximum exercise heart rate decreases with maturation in the rat as well as
in man. The present study was undertaken to investigate the mechanism(s)
which might be responsible for this reduction in max exercise heart rate in
the rat. Maximum exercise heart rates were 618 +/- 7 vs. 580 +/- 9
beats/min for the young (5 wk) and mature (19 wk) rats, respectively.
Atropine had no effect on max exercise heart rate. Propranolol reduced max
heart rate in both groups with the older rats having the lowest value.
Resting heart rates were recorded following injections of atropine
propranolol, and propranolol plus atropine. Under all conditions, including
control, heart rates were lower in the mature rats. Electrical stimulation
in situ showed a significant difference in the threshold voltage for
stimulation at 600 beats/min: 1.1 +/- 0.2 vs. 1.6 +/- 0.1 V for the young
and mature rats, respectively. These data suggest that intrinsic changes
occur in the myocardium with maturation and the reduction in max exercise
heart rate is due to these intrinsic changes as opposed to changes in
neural influences.
